Since the first world war the threat of poisonous airborne agents has become a public concern. Further awareness regarding health hazards borne by the air is currently on the rise because of the general environmental conditions and occasional disasters caused by industrial exposures and accidental leaks.
Airborne hazardous agents are either true gasses or aerosols, containing noxious molecules or microorganisms. They penetrate the human body chiefly via the respiratory tract. The exposure of the upper and lower airways and the lung alveoli to toxic gases cause severe local and systemic reactions that rapidly lead to incapacitation and death. To prevent exposure of the lungs to toxic gases and biological agents, a variety of protective gear and barriers have been developed. In principle these devices include an impermeable shield around the face, the head, or the whole body, and means of supplying fresh and decontaminated air to sustain pulmonary gas exchange. The protective gear that is currently available may be subdivided into two primary classes: passive and active.
Passive protection refers to gas masks where the flow of air into the mask through the filter is driven by the kinetic energy provided by the user's own inspiratory system. Active protection refers to masks and other shields where the flow into the breathing circuit is driven by a pump or a blower. Active protection has the distinct advantages of being more effective by preventing penetration of the toxic gases through cracks or incomplete seal between the mask and the skin. This is achieved by running excess flow into the breathing circuit and maintaining positive pressure inside the shield at all times. Active protection gear is also safer since it assures, at all times, ample supply of fresh air, free of carbon dioxide and rich in oxygen. Additional advantage of active gear is the avoidance of excessive negative pressures needed to generate flow through the filter, especially during strenuous activity, when peak inspiratory flow is elevated.
The disadvantages of active protective gear are their higher cost, their reliance on a power source such as batteries, their increased weight, their susceptibility to breakage and malfunction and their complexity. Of particular concern are the durability and shelf life of the batteries. The invention disclosed herewith is of a gas mask that is primarily of the passive type.
Additional classification of protective gear relates to the location of the seal in relation to the user's skin. There are four types of mask seals: 1) the face mask, sealed around the user's face; 2) the mouth-nose mask, sealed only around the respiratory inlets; 3) the hood protective gear that is sealed around the neck; and 4) the hybrid double seal device that is constructed from an external hood or face mask and an inner mouth-nose compartment that is sealed around the mouth and nose.
The face type gas mask is the standard model used by the military, such as the M40 used by the US army. It covers the mouth, nose and eyes and has a relatively large dead space. The seal of this mask is around the face from the forehead around the maxillae and cheeks down to below the chin.
This seal of the facemask must fit snugly to prevent leakage of toxic gases into the mask. This seal is not applicable in specific cases. For example beard wearers, or individuals with unusual facial proportions or deformations. In addition, the facemask requires individual sizing and meticulous adjustment of the pull-straps that hold the mask to the face. A common variant of the facemask, such as disclosed by Grove, Chase and Fritch in U.S. Pat. No. 6,176,239, includes an additional mouth-nose compartment inside the face mask. This compartment is also intended to fit snugly around the nose and mouth to reduce re-breathing of carbon dioxide and fogging of the mask lenses.
The mouth-nose mask only protects the user's respiratory inlet and requires separate goggles to protect the eyes. As such, it is not suitable for general protection against chemical and biological warfare, but is often used in industry where light weight and convenience are important and the level of exposure risk is well-known in advance.
The third type is the hood protection gear that is sealed around the user's neck. This hood can only be used as an active mask because of its large dead space and compliant walls that promote carbon dioxide retention if a pump or blower is not used.
The fourth type is the hybrid double seal device. Such a mask includes a mouth-nose compartment that fits the user's face snugly and an enclosing hood fitted with a visor and a membrane sealing it around the user's neck. This type of gear provides a high protection ratio, but is only safe to use if the mouth-nose compartment is tightly sealed. If the seal is incomplete or breaks as a result of head motion, speech or other movements, carbon dioxide-rich exhaled gas escapes into the hood cavity and is re-breathed during the subsequent inhalation. Such re-breathing may cause carbon dioxide build-up and suffocation.
Further classification of gas masks relates to the different user grouping. In essence, there are two subgroups: 1) masks intended for active personnel such as emergency crews and military personnel and 2) masks intended for sedentary civilian population. There are differences in design and construction as well as in distribution strategy for the two subgroups. The masks for civilian population are usually distributed with only rough individual customization (i.e., ‘large’, ‘medium’, ‘small’). They must be very simple to use with only minimal training (eg., a video tutoring film). Moreover, the system must be both effective and safe beyond doubt for the vast majority of people. Thus, it should provide adequate protection while being safe under multitude of circumstances. Safety criteria include upper threshold for inhaled carbon dioxide level (F1CO2) that must be less than 2% and a minimum value of inhaled oxygen (F1O2) that must be greater than 17%. These thresholds must not be violated for a period that is continuously longer than sixty seconds. The novel chemical-biological protection gear disclosed herewith is specifically intended for use by such untrained, diverse civilian populations.
The final subdivision of protective gear is by the placement and configuration of the inlet and outlet respiratory valves. In all gas masks the exhalation outlet forms a direct communication with the mouth-nose compartment so that the exhaled gas can exit the system easily and promptly.
The placement of the inhalation valve(s) varies among gas masks, but in most systems including an inner mouth-nose compartment, there are two sets of inspiratory valves: 1) a valve leading from the filter into the cavity of the outer shell and 2) a valve or valves leading from the outer shell into the mouth-nose compartment. The advantage of this configuration is that fresh air flushes the interior of the outer shell with every breath. This is most useful in keeping moisture from condensing on the lenses or visor elements of the mask. The disadvantage of this arrangement is that exhaled gas that may escape under the seal from the mouth-nose compartment into the shell mixes with the fresh gas and causes partial re-breathing of a carbon dioxide-rich and oxygen-poor gas.
Placement of the inhalation valve directly in communication with the mouth-nose compartment is found in industrial mouth-nose masks where independent goggles are used for eye protection. When used in a facemask variant that does not have a sealed mouth-nose compartment, the internal volume becomes too large (i.e., 500 ml.) creating an excessive respiratory dead space, which may be too large for persons with small lung capacity.